The present invention relates generally to gas turbine engines, and more particularly to fuel systems for auxiliary power units.
Large commercial aircraft typically include on-board auxiliary power units (APUs), located in the tail sections of the aircraft, to provide electrical power and compressed air for systems throughout the aircraft. When an aircraft is on the ground, the primary propulsion engines of the aircraft are shut down, and the auxiliary power unit provides the main source of power for a variety of systems, such as the environmental control systems, hydraulic pumps, electrical systems, and main engine starters. The auxiliary power unit may also provide power during in-flight operations, such as for electrical and pneumatic systems.
Low temperatures associated with high altitudes can pose difficulties to in-flight startup and operation of traditional APUs. Low fuel temperatures can result in increased fuel viscosity, causing fuel injected into an APU combustor to form larger-than-usual droplets. The reduced area/volume ratio of such droplets reduces fuel vaporization required for combustion ignition, thereby impeding APU operability. Simultaneously, low air temperatures present a larger-than-usual temperature gap that must be bridged to reach fuel ignition temperatures. In combination, low fuel and air temperatures can prevent or delay APU starting, impair sustained APU operability, and reduce the fuel efficiency of APU operation.